While performing parity
experiments it is vital to prevent electronic cross-talk that could feed real-time
helicity information to the experimenter.Ground loops could transmit this signal and must be eliminated.

When we developed the original
Pockels Cell driver for CEBAF, parity experiments were not at the front of our
mind.We were a bit careless with
the layout of our electronics.This
is evidenced by the following schematic layout:

The errors should be obvious, but I will point them out.

The
DAC’s that program the 4000V DC-DC converters have a common ground
reference with the VME crate in the injector service building.

The 24VDC bulk supply for the DC-DC converters
has a ground reference in the injector service building.

The Helicity generator chassis was in the same
rack upstairs, sharing power with the VME crate electronics.

Not shown on the drawing—A 1000:1 high
voltage probe would monitor the voltage on the pockels cell as a safety
precaution to warn the halls if the flipping stopped.The signal was buffered and sent
into two ADC channels.Fundamentally it was a good idea, but only if it is implemented in
such a way as to not pass real-time helicity information.It shared a common ground, so it
was a suspect for passing information.

These grounds were all in common with a distant ground on
the Pockels cell and 4000V dc-dc converters.Every time the cell would switch, the
grounds could all take a small jump as they bleed down the current that passed
through the high voltage switches.

These errors were corrected in 2004 by isolating the system
grounds to a common ground in the tunnel as shown below.

All system commons are designated by the blue lines.All commons join earth ground at a
single point.The helicity
generator is located in a floating rack and the helicity information is fed on
fiber to the high voltage switch.The DAC programming voltages are optically isolated, so there is no
possibility of transferring helicity information back up to the injector
service building.

The remote voltage monitoring has been eliminated for
now.The halls tell us that their
data would tell them almost immediately if there was an unexpected loss of
helicity flipping.

2007 plans

In the summer of 2007 the wiring to the laser room will be
cleaned up.All information for
programming and readback of signals related to helicity components (Pockels
Cell and IA’s) will pass through a new isolator card.This card uses the ISO-124
chip to isolate signals from -10V to +10V with a bandwidth of up to 50kHz.The card we have designed will isolate 8
channels from the service building down to the laser room, and 4 channels from
the laser room back up to the service building.We will have 3 of these cards in
place.We will need to ensure that
any signal that is placed on a channel from tunnel to service building is
heavily filtered with at least a 100ms time constant filter to ensure there is
no real time helicity information being passed.

January 2008:

The helicity system is configured as shown in the figure
below:

All circuits seeing real time helicity are floating with
respect to earth ground.The option
remains to connect a single point ground if requested by the researchers.Thus far we have had no complaints about
unintended helicity pickup.

April 2010:

A “quad-IA” system has been developed that will
permit small corrections to the IA cell for Hall C on a quartet basis.The system is configured as shown in the
figure below: